Synthesis and evaluation of water-soluble polymeric bone-targeted drug delivery systems

Development of tetracycline-modified nanoparticles for bone-targeted delivery of anti-tubercular drug

Background: Since the poor response to existing anti-tuberculosis drugs and low drug concentration in local bone tissues, the traditional drug therapy does not result in satisfactory treatment of osteoarticular tuberculosis. Thus, we report a rifapentine release system with imparted bone targeting potential using tetracycline (TC) -modified nanoparticles (NPs). Methods: TC was conjugated to PLGA-PEG copolymer via a DCC/NHS technique. Rifapentine-loaded NPs were prepared by premix membrane emulsification technique. The resulting NPs were characterized in terms of physicochemical characterization, hemolytic study, cytotoxicity, bone mineral binding ability, in vitro drug release, stability test and antitubercular activity. The pharmacokinetic and biodistribution studies were also performed in mice. Results: Rifapentine loaded TC-PLGA-PEG NPs were proved to be 48.8 nm in size with encapsulation efficiency and drug loading of 83.3% ± 5.5% and 8.1% ± 0.4%, respectively. The release of rifapentine from NPs could be maintained for more than 60 h. Most (68.0%) TC-PLGA-PEG NPs could bind to HAp powder in vitro. The cellular studies revealed that NPs were safe for intravenous administration. In vivo evaluations also revealed that the drug concentration of bone tissue in TC-PLGA-PEG group was significantly higher than that in other groups at all time (p < 0.05). Both NPs could improve pharmacokinetic parameters without evident organ toxicity. The minimal inhibitory concentration of NPs was 0.094 μg/mL, whereas this of free rifapentine was 0.25 μg/mL. Conclusion: Rifapentine loaded TC-PLGA-PEG NPs could increase the amount of rifapentine in bone tissue, prolong drug release in systemic circulation, enhance anti-tuberculosis activity, and thereby reducing dose and frequency of drug therapy for osteoarticular tuberculosis.

Pyrophosphorylated-Cholesterol-Modified Bone-Targeting Liposome Formulation Procedure

Bone-targeting drug delivery systems have been rapidly developed to increase drug efficacy and safety for musculoskeletal diseases in the past decades. Bone-targeting drug delivery is mainly based on ligands that have hydroxyapatite affinity. We previously reported a pyrophosphorylated cholesterol ligand-based bone-targeting liposome formulation for the treatment of bone fracture delayed union. Different from traditional bone-targeting ligands: bisphosphonates tetracyclines and polyanion peptides. Pyrophosphorylated cholesterol has no intrinsic pharmacological effects and can be naturally degraded into metabolites (both pyrophosphate and cholesterol are substances that naturally exist in the body), leading to minimal safety concerns. Pyrophosphorylated cholesterol is not only biodegradable, but it also provides strong bone affinity, which could target different bone substructures/surfaces, further improving drug delivery efficiency in vivo. Here, we describe the synthesis protocol of pyrophosphorylated cholesterol and a reverse-evaporation-based formulation protocol of pyrophosphorylated-cholesterol-modified bone-targeting liposomes for hydrophilic drug encapsulation. We also provide instructions for the bone-targeting property evaluation of the pyrophosphorylated-cholesterol-modified liposome in vitro and in vivo. Our system has wide applications and has already been used to study drug treatment for fracture delayed union and nonunion. As a promising bone-targeting drug delivery system, our system may be extrapolated to clinical applications of other bone anabolic agents for different bone diseases.

Drug Delivery to the Bone Microenvironment Mediated by Exosomes: An Axiom or Enigma

The increasing incidence of bone-related disorders is causing a burden on the clinical scenario. Even though bone is one of the tissues that possess tremendous regenerative potential, certain bone anomalies need therapeutic intervention through appropriate delivery of a drug. Among several nanosystems and biologics that offer the potential to contribute towards bone healing, the exosomes from the class of extracellular vesicles are outstanding. Exosomes are extracellular nanovesicles that, apart from the various advantages, are standing out of the crowd for their ability to conduct cellular communication. The internal cargo of the exosomes is leading to its potential use in therapeutics. Exosomes are being unraveled in terms of the mechanism as well as application in targeting various diseases and tissues. Through this review, we have tried to understand and review all that is already established and the gap areas that still exist in utilizing them as drug delivery vehicles targeting the bone. The review highlights the potential of the exosomes towards their contribution to the drug delivery scenario in the bone microenvironment. A comparison of the pros and cons of exosomes with other prevalent drug delivery systems is also done. A section on the patents that have been generated so far from this field is included.

The evolution of polymer conjugation and drug targeting for the delivery of proteins and bioactive molecules

Polymer conjugation can be considered one of the leading approaches within the vast field of nanotechnology-based drug delivery systems. In fact, such technology can be exploited for delivering an active molecule, such as a small drug, a protein, or genetic material, or it can be applied to other drug delivery systems as a strategy to improve their in vivo behavior or pharmacokinetic activities such as prolonging the half-life of a drug, conferring stealth properties, providing external stimuli responsiveness, and so on. If on the one hand, polymer conjugation with biotech drug is considered the linchpin of the protein delivery field boasting several products in clinical use, on the other, despite dedicated research, conjugation with low molecular weight drugs has not yet achieved the milestone of the first clinical approval. Some of the primary reasons for this debacle are the difficulties connected to achieving selective targeting to diseased tissue, organs, or cells, which is the main goal not only of polymer conjugation but of all delivery systems of small drugs. In light of the need to achieve better drug targeting, researchers are striving to identify more sophisticated, biocompatible delivery approaches and to open new horizons for drug targeting methodologies leading to successful clinical applications. This article is categorized under: Therapeutic Approaches and Drug Discovery > Emerging Technologies Therapeutic Approaches and Drug Discovery > Nanomedicine for Oncologic Disease Toxicology and Regulatory Issues in Nanomedicine > Regulatory and Policy Issues in Nanomedicine.

Targeting nanoparticles for diagnosis and therapy of bone tumors: Opportunities and challenges

A variety of targeted nanoparticles were developed for the diagnosis and therapy of orthotopic and metastatic bone tumors during the past decade. This critical review will focus on principles and methods in the design of these bone-targeted nanoparticles. Ligands including bisphosphonates, aspartic acid-rich peptides and synthetic polymers were grafted on nanoparticles such as PLGA nanoparticles, liposomes, dendrimers and inorganic nanoparticles for bone targeting. Besides, other ligands such as monoclonal antibodies, peptides and aptamers targeting biomarkers on tumor/bone cells were identified for targeted diagnosis and therapy. Examples of targeted nanoparticles for the early detection of bone metastatic tumors and the ablation of cancer via chemotherapy, photothermal therapy, gene therapy and combination therapy will be intensively reviewed. The development of multifunctional nanoparticles to break down the "vicious" cycle between tumor cell proliferation and bone resorption, and the challenges and perspectives in this area will be discussed.

Identification of osteogenic progenitor cell-targeted peptides that augment bone formation

Activation and migration of endogenous mesenchymal stromal cells (MSCs) are critical for bone regeneration. Here, we report a combinational peptide screening strategy for rapid discovery of ligands that not only bind strongly to osteogenic progenitor cells (OPCs) but also stimulate osteogenic cell Akt signaling in those OPCs. Two lead compounds are discovered, YLL3 and YLL8, both of which increase osteoprogenitor osteogenic differentiation in vitro. When given to normal or osteopenic mice, the compounds increase mineral apposition rate, bone formation, bone mass, and bone strength, as well as expedite fracture repair through stimulated endogenous osteogenesis. When covalently conjugated to alendronate, YLLs acquire an additional function resulting in a “tri-functional” compound that: (i) binds to OPCs, (ii) targets bone, and (iii) induces “pro-survival” signal. These bone-targeted, osteogenic peptides are well suited for current tissue-specific therapeutic paradigms to augment the endogenous osteogenic cells for bone regeneration and the treatment of bone loss.

Activation of osteogenic cells is essential for bone regeneration. Here, the authors screen a peptide library and identify 2 compounds that promote osteogenic progenitor cell differentiation in vitro, and show that they increase bone formation and fracture repair in mice.

Bone-Targeting Liposome-Encapsulated Salvianic Acid A Improves Nonunion Healing Through the Regulation of HDAC3-Mediated Endochondral Ossification

AIM

Nonunion is a major complication in fracture repair and remains a challenge in orthopaedics and trauma surgery. In this study, we aimed to evaluate the effectiveness of treatment of nonunion with a large radial defect using a bone-targeting liposome-encapsulated salvianic acid A (SAA-BTL)-incorporated collagen sponge and further elucidate whether the effects were closely related to histone deacetylase 3 (HDAC 3)-mediated endochondral ossification in nonunion healing process.

METHODS

Fifteen New Zealand female rabbits were randomly divided into three groups. Segmental radius critical size defects (15 mm) were created via surgery on both the forelimbs of the rabbits. The SAA-BTL/SAA/saline-incorporated collagen sponges were implanted into the defects in the three groups, respectively, for four weeks of treatment. X-ray imaging, micro-computed tomography (CT) analysis, histology, and immunofluorescence analysis (HDAC3, collagen II, VEGFA, and osteocalcin) were performed to determine the effects of the treatments. In addition, a short interfering RNA was applied to induce HDAC3 knockdown in the chondrogenic cell line ATDC5 to investigate the roles of HDAC3 and SAA intervention in endochondral ossification in nonunion healing.

RESULTS

X-ray imaging and micro-CT results revealed that SAA-BTL-incorporated collagen sponges significantly stimulated bone formation in the nonunion defect rabbit model. Furthermore, immunofluorescence double staining and histology analysis confirmed that SAA-BTL significantly increased the expression of P-HDAC3, collagen II, RUNX2, VEGFA, and osteocalcin in vivo; accelerated endochondral ossification turnover from cartilage to bone; and promoted long bone healing of nonunion defects. ATDC5 cells knocked down for HDAC3 showed significantly decreased expression of HDAC3, which resulted in reduced expression of chondrogenesis, osteogenesis, and angiogenesis biomarker genes (Sox9, Col10a1, VEGFA, RUNX2, and Col1a1), and increased expression of extracellular matrix degradation marker (MMP13). SAA treatment reversed these effects in the HDAC3 knockdown cell model.

CONCLUSION

SAA-BTL can improve nonunion healing through the regulation of HDAC3-mediated endochondral ossification.

Molecular insights and novel approaches for targeting tumor metastasis

In recent years, due to the effective drug delivery and preciseness of tumor sites or microenvironment, the targeted drug delivery approaches have gained ample attention for tumor metastasis therapy. The conventional treatment approaches for metastasis therapy have reported with immense adverse effects because they exhibited maximum probability of killing the carcinogenic cells along with healthy cells. The tumor vasculature, comprising of vasculogenic impressions and angiogenesis, greatly depends upon the growth and metastasis in the tumors. Therefore, various nanocarriers-based delivery approaches for targeting to tumor vasculature have been attempted as efficient and potential approaches for the treatment of tumor metastasis and the associated lesions. Furthermore, the targeted drug delivery approaches have found to be most apt way to overcome from all the limitations and adverse effects associated with the conventional therapies. In this review, various approaches for efficient targeting of pharmacologically active chemotherapeutics against tumor metastasis with the cohesive objectives of prognosis, tracking and therapy are summarized.

Cathepsin-sensitive nanoscale drug delivery systems for cancer therapy and other diseases

Cathepsins are an important category of enzymes that have attracted great attention for the delivery of drugs to improve the therapeutic outcome of a broad range of nanoscale drug delivery systems. These proteases can be utilized for instance through actuation of polymer-drug conjugates (e.g., triggering the drug release) to bypass limitations of many drug candidates. A substantial amount of work has been witnessed in the design and the evaluation of Cathepsin-sensitive drug delivery systems, especially based on the tetra-peptide sequence (Gly-Phe-Leu-Gly, GFLG) which has been extensively used as a spacer that can be cleaved in the presence of Cathepsin B. This Review Article will give an in-depth overview of the design and the biological evaluation of Cathepsin-sensitive drug delivery systems and their application in different pathologies including cancer before discussing Cathepsin B-cleavable prodrugs under clinical trials.

Biomaterial-assisted local and systemic delivery of bioactive agents for bone repair

Although bone tissues possess an intrinsic capacity for repair, there are cases where bone healing is either impaired or insufficient, such as fracture non-union, osteoporosis, osteomyelitis, and cancers. In these cases, treatments like surgical interventions are used, either alone or in combination with bioactive agents, to promote tissue repair and manage associated clinical complications. Improving the efficacy of bioactive agents often requires carriers, with biomaterials being a pivotal player. In this review, we discuss the role of biomaterials in realizing the local and systemic delivery of biomolecules to the bone tissue. The versatility of biomaterials enables design of carriers with the desired loading efficiency, release profile, and on-demand delivery. Besides local administration, systemic administration of drugs is necessary to combat diseases like osteoporosis, warranting bone-targeting drug delivery systems. Thus, chemical moieties with the affinity towards bone extracellular matrix components like apatite minerals have been widely utilized to create bone-targeting carriers with better biodistribution, which cannot be achieved by the drugs alone. Bone-targeting carriers combined with the desired drugs or bioactive agents have been extensively investigated to enhance bone healing while minimizing off-target effects. Herein, these advancements in the field have been systematically reviewed. STATEMENT OF SIGNIFICANCE: Drug delivery is imperative when surgical interventions are not sufficient to address various bone diseases/defects. Biomaterial-assisted delivery systems have been designed to provide drugs with the desired loading efficiency, sustained release, and on-demand delivery to enhance bone healing. By surveying recent advances in the field, this review outlines the design of biomaterials as carriers for the local and systemic delivery of bioactive agents to the bone tissue. Particularly, biomaterials that bear chemical moieties with affinity to bone are attractive, as they can present the desired bioactive agents to the bone tissue efficiently and thus enhance the drug efficacy for bone repair.

Development of Zoledronic Acid-Based Nanoassemblies for Bone-Targeted Anticancer Therapy

Bone metastasis occurs in the majority of cancer patients, which hampers quality of life and significantly decreases survival. Aggressive chemotherapy is a traditional treatment regimen that induces severe systemic toxicities. Therefore, bone-directed therapies are highly warranted. We report a novel nanoparticle formulation that is composed of poly(vinylpyrrolidone) and tannic acid core nanoparticles (PVT NPs) that forms self-assembly with zoledronic acid (ZA@PVT NPs). The construction of ZA@PVT NPs was confirmed by particle size, zeta potential, transmission electron microscopy, and spectral analyses. An optimized bone-targeted ZA@PVT NPs formulation showed greater binding and internalization in in vitro with metastasis prostate and breast cancer cells. ZA@PVT NPs were able to deliver ZA more efficiently to tumor cells, which inhibited proliferation of human prostate and breast cancer cells. In addition, ZA@PVT NPs were capable of targeting mouse bones and prostate tumor microarray tissues (ex vivo) while sparing all other vital organs. More importantly, ZA@PVT NPs induce chemo sensitization to docetaxel treatment in cancer cells. Overall, the study results confirm that ZA-based, bone-targeted NPs have great potential for the treatment of bone metastasis in the near future.

Bone-targeting carbon dots: effect of nitrogen-doping on binding affinity

Novel fluorescent carbon dots (CDs) for bone imaging were fabricated via a facile hydrothermal method using alendronate in the absence of a nitrogen-doping precursor to enhance bone affinity. One-step synthesized alendronate-based CDs (Alen-CDs) had strong binding activity for calcium-deficient hydroxyapatite (CDHA, the mineral component of bones) scaffold, rat femur, and bone structures of live zebrafish. This was attributed to the bisphosphonate group present on the CD surface, even after carbonization. For comparison, the surface effects of nitrogen-doped CDs obtained using ethylenediamine (EDA), i.e., Alen-EDA-CDs, were also investigated, focusing on the targeting ability of distinct surface functional groups when compared with Alen-CDs. An in vivo study to assess the impact on bone affinity revealed that Alen-CDs effectively accumulated in the bone structures of live zebrafish larvae after microinjections, as well as in the bone tissues of femur extracted from rats. Moreover, Alen-CD-treated zebrafish larvae had superior toleration, retaining skeletal fluorescence for 7 days post-injection (dpi). The sustainable capability, surpassing that of Alizarin Red S, suggests that Alen-CDs have the potential for targeted drug delivery to damaged bone tissues and provides motivation for additional in vivo investigations. To our knowledge, this is the first in vitro, ex vivo, and in vivo demonstration of direct bone-targeted deliveries, supporting the use of fluorescent CDs in the treatment of various bone diseases such as osteoporosis, Paget's disease, and metastatic bone cancer.

Fluorescent carbon dots selectively bind to skull tissues with high affinity, including a strong binding activity for calcium deficient hydroxyapatite, and rat femur, for bone targeted imaging.

Emerging biomedical applications of polyaspartic acid-derived biodegradable polyelectrolytes and polyelectrolyte complexes

A summary of positive biomedical attributes of biodegradable polyelectrolytes (PELs) prepared from aspartic acid is provided. The utility of these PELs in emerging applications such as biomineralization modulators, antimycobacterials, biocompatible cell encapsulants and tissue adhesives is highlighted.

Development of Diagnostic and Therapeutic Probes with Controlled Pharmacokinetics for Use in Radiotheranostics

The word "theranostics," a portmanteau word made by combining "therapeutics" and "diagnostics," refers to a personalized medicine concept. Recently, the word, "radiotheranostics," has also been used in nuclear medicine as a term that refer to the use of radioisotopes for combined imaging and therapy. For radiotheranostics, a diagnostic probe and a corresponding therapeutic probe can be prepared by introducing diagnostic and therapeutic radioisotopes into the same precursor. These diagnostic and therapeutic probes can be designed to show equivalent pharmacokinetics, which is important for radiotheranostics. As imaging can predict the absorbed radiation dose and thus the therapeutic and side effects, radiotheranostics can help achieve the goal of personalized medicine. In this review, I discuss the use of radiolabeled probes targeting bone metastases, sigma-1 receptor, and αVβ3 integrin for radiotheranostics.

Bone-targeting poly(ethylene sodium phosphate)

Poly(ethylene sodium phosphate) (PEP·Na) showed excellent cytocompatibility and in vivo bone affinity. Moreover, PEP·Na did not interact with thrombin, which is a coagulation-related protein. Because immobilization of therapeutic agents and imaging probes on PEP·Na is easily performed, PEP·Na is a promising polymer for bone-targeted therapies.

Development of zoledronic acid functionalized hydroxyapatite loaded polymeric nanoparticles for the treatment of osteoporosis

The present study formulated a hydroxyapatite (HA)-coated lipid nanoparticle loaded with zoledronic acid to treat patients with osteoporosis (OP). HA-coated zoledronic acid loaded lipid bilayer nanoparticles (HZL NPs) were prepared using methods of extrusion and precipitation. Nanosized particles were prepared with the aim of increasing gradual and prolonged drug release and inducing toxicity of osteoblasts. Cellular morphology was investigated by scanning electron microscopy, which revealed clear spherical shaped NPs ~200 nm in size that could treat osteopores in the bone. In addition, a typical biphasic release pattern was observed that could be attributed to the presence of drug on the outer surface as well as on the inner core of the NPs. There was sustained release behavior [38.17±2.12% (pH 7.4) and 64.2±3.75% (pH 5) at 48 h] of the drug that maintained the drug reservoir effect at the bone site. Furthermore, HZL NPs increased the cytotoxicity of HFOb 1.19 cells and increased the proportion of cells in the early (18.1±12.4%), late (28.7±3.7%) and necrotic (67.5±1.2%) phases of apoptosis. Most importantly, HZL in the lipid nanoparticle exhibited a strong affinity towards HA, further enhancing its efficacy in the treatment of OP.

Concise Review: Fabrication, Customization, and Application of Cell Mimicking Microparticles in Stem Cell Science

Stem and non-stem cell behavior is heavily influenced by the surrounding microenvironment, which includes other cells, matrix, and potentially biomaterials. Researchers have been successful in developing scaffolds and encapsulation techniques to provide stem cells with mechanical, topographical, and chemical cues to selectively direct them toward a desired differentiation pathway. However, most of these systems fail to present truly physiological replications of the in vivo microenvironments that stem cells are typically exposed to in tissues. Thus, cell mimicking microparticles (CMMPs) have been developed to more accurately recapitulate the properties of surrounding cells while still offering ways to tailor what stimuli are presented. This nascent field holds the promise of reducing, or even eliminating, the need for live cells in select, regenerative medicine therapies, and diagnostic applications. Recent, CMMP-based studies show great promise for the technology, yet only reproduce a small subset of cellular characteristics from among those possible: size, morphology, topography, mechanical properties, surface molecules, and tailored chemical release to name the most prominent. This Review summarizes the strengths, weaknesses, and ideal applications of micro/nanoparticle fabrication and customization methods relevant to cell mimicking and provides an outlook on the future of this technology. Moving forward, researchers should seek to combine multiple techniques to yield CMMPs that replicate as many cellular characteristics as possible, with an emphasis on those that most strongly influence the desired therapeutic effects. The level of flexibility in customizing CMMP properties allows them to substitute for cells in a variety of regenerative medicine, drug delivery, and diagnostic systems. Stem Cells Translational Medicine 2018;7:232-240.

Multivalent Presentation of Peptide Targeting Groups Alters Polymer Biodistribution to Target Tissues

Drug delivery to bone is challenging, whereby drug distribution is commonly <1% of injected dose, despite development of several bone-targeted drug delivery systems specific to hydroxyapatite. These bone-targeted drug delivery systems still suffer from poor target cell localization within bone, as at any given time overall bone volume is far greater than acutely remodeling bone volume, which harbors relevant cell targets (osteoclasts or osteoblasts). Thus, there exists a need to target bone-acting drugs specifically to sites of bone remodeling. To address this need, this study synthesized oligo(ethylene glycol) copolymers based on a peptide with high affinity to tartrate-resistant acid phosphatase (TRAP), an enzyme deposited by osteoclasts during the bone resorption phase of bone remodeling, which provides greater specificity relevant for bone cell drugging. Gradient and random peptide orientations, as well as polymer molecular weights, were investigated. TRAP-targeted, high molecular weight (M_n) random copolymers exhibited superior accumulation in remodeling bone, where fracture accumulation was observed for at least 1 week and accounted for 14% of tissue distribution. Intermediate and low M_n random copolymer accumulation was lower, indicating residence time depends on M_n. High M_n gradient polymers were cleared, with only 2% persisting at fractures after 1 week, suggesting TRAP binding depends on peptide density. Peptide density and M_n are easily modified in this versatile targeting platform, which can be applied to a range of bone drug delivery applications.

Intended and Unintended Targeting of Polymeric Nanocarriers: The Case of Modified Poly(glycerol adipate) Nanoparticles

Biodegradable nanoparticles based on stearic acid-modified poly(glycerol adipate) (PGAS) are promising carriers for drug delivery. In order to investigate the impact of the particle interface characteristics on the biological fate, PGAS nanoparticles are covalently and noncovalently coated with N-(2-hydroxypropyl) methacrylamide (HPMA) copolymers. HPMA copolymer-modified PGAS nanoparticles have similar particle sizes, but less negative zeta-potentials. Nanoparticles are double labeled with the fluorescent dyes DiR (noncovalently) and DYOMICS-676 (covalently bound to HPMA copolymer), and their biodistribution is investigated noninvasively by multispectral optical imaging. Both covalent and noncovalent coatings cause changes in the pharmacokinetics and biodistribution in healthy and tumor-bearing mice. In addition to the intended tumor accumulation, high signals of both fluorescent dyes are also observed in other organs, including liver, ovaries, adrenal glands, and bone. The unintended accumulation of nanocarriers needs further detailed and systematic investigations, especially with respect to the observed ovarian and adrenal gland accumulation.

Evaluation of Ga-DOTA-(D-Asp)n as bone imaging agents: D-aspartic acid peptides as carriers to bone

⁶⁷Ga-DOTA-(L-Asp)₁₁ and ⁶⁷Ga-DOTA-(L-Asp)₁₄, which have been developed as bone imaging agents, showed a high accumulation in bone and a rapid blood clearance in mice. However, peptides composed of D-amino acids are more stable in vivo than those composed of their L-equivalents. In this study, ⁶⁷Ga-DOTA-(D-Asp)_n (n = 2, 5, 8, 11, or 14) were synthesized using the Fmoc-based solid-phase methodology and evaluated. In hydroxyapatite binding assay, binding of ⁶⁷Ga-DOTA-(D-Asp)_n tended to increase with increasing length of the amino acid chain. ⁶⁷Ga-DOTA-(D-Asp)₁₁ and ⁶⁷Ga-DOTA-(D-Asp)₁₄ caused a high accumulation of radioactivity in the bones of the mice. However, the results for ⁶⁷Ga-DOTA-(D-Asp)_n and ⁶⁷Ga-DOTA-(L-Asp)_n were comparable. In urine analyses, the proportion of intact complex after injection of ⁶⁷Ga-DOTA-(D-Asp)₁₄ was significantly higher than that of ⁶⁷Ga-DOTA-(L-Asp)₁₄. Although ⁶⁷Ga-DOTA-(D-Asp)₁₄ was more stable than ⁶⁷Ga-DOTA-(L-Asp)₁₄, the properties of ⁶⁷Ga-DOTA-(D-Asp)_n and ⁶⁷Ga-DOTA-(L-Asp)_n as bone imaging agents may be comparable.

Design of polyaspartic acid peptide-poly (ethylene glycol)-poly (ε-caprolactone) nanoparticles as a carrier of hydrophobic drugs targeting cancer metastasized to bone

Treatment of cancer metastasized to bone is still a challenge due to hydrophobicity, instability, and lack of target specificity of anticancer drugs. Poly (ethylene glycol)-poly (ε-caprolactone) polymer (PEG-PCL) is an effective, biodegradable, and biocompatible hydrophobic drug carrier, but lacks bone specificity. Polyaspartic acid with eight peptide sequences, that is, (Asp)₈, has a strong affinity to bone surface. The aim of this study was to synthesize (Asp)₈-PEG-PCL nanoparticles as a bone-specific carrier of hydrophobic drugs to treat cancer metastasized to bone. ¹H nuclear magnetic resonance, Fourier transform infrared spectroscopy, and transmission electron microscopy data showed that (Asp)₈-PEG-PCL nanoparticles (size 100 nm) were synthesized successfully. (Asp)₈-PEG-PCL nanoparticles did not promote erythrocyte aggregation. Fluorescence microscopy showed clear uptake of Nile red-loaded (Asp)₈-PEG-PCL nanoparticles by cancer cells. (Asp)₈-PEG-PCL nanoparticles did not show cytotoxic effect on MG63 and human umbilical vein endothelial cells at the concentration of 10-800 μg/mL. (Asp)₈-PEG-PCL nanoparticles bound with hydroxyapatite 2-fold more than PEG-PCL. Intravenously injected (Asp)₈-PEG-PCL nanoparticles accumulated 2.7-fold more on mice tibial bone, in comparison to PEG-PCL. Curcumin is a hydrophobic anticancer drug with bone anabolic properties. Curcumin was loaded in the (Asp)₈-PEG-PCL. (Asp)₈-PEG-PCL showed 11.07% loading capacity and 95.91% encapsulation efficiency of curcumin. The curcumin-loaded (Asp)₈-PEG-PCL nanoparticles gave sustained release of curcumin in high dose for >8 days. The curcumin-loaded (Asp)₈-PEG-PCL nanoparticles showed strong antitumorigenic effect on MG63, MCF7, and HeLa cancer cells. In conclusion, (Asp)₈-PEG-PCL nanoparticles were biocompatible, permeable in cells, a potent carrier, and an efficient releaser of hydrophobic anticancer drug and were bone specific. The curcumin-loaded (Asp)₈-PEG-PCL nanoparticles showed strong antitumorigenic ability in vitro. Therefore, (Asp)₈-PEG-PCL nanoparticles could be a potent carrier of hydrophobic anticancer drugs to treat the cancer metastasized to bone.

Targeting to the Bone: Alendronate-Directed Combretastatin A-4 Bearing Antiangiogenic Polymer-Drug Conjugates

Selective targeting of tumor site with chemotherapeutic agents appears to be one of the most effective methods to address many of the problems encountered with conventional chemotherapy. In this work, poly(oligoethylene glycol)methacrylate (POEGMA) based bone-targeting polymers bearing an antiangiogenic drug combretastatin A4 (CA4) were synthesized using free radical polymerization. Targeted and nontargeting copolymers were evaluated for their bone targeting efficiency, cytotoxicities against endothelial cells, namely, HUVECs and U2-OS and Saos-2 cancerous cell lines, as well as their antiangiogenic activity against endothelial cell tube formation by HUVECs. It is observed that the drug conjugated polymers conjugated with the bisphosphonate groups containing drug alendronate (ALN) have remarkably high affinity for bone mineral when compared to the polymer-drug conjugates devoid of the bisphosphonate groups. Both targeted and nontargeted polymer-drug conjugates show a sustained drug release in rat plasma with an overall release of 80-93% over 5 days. In vitro studies revealed high levels of cytotoxicity of the polymer-drug conjugates against HUVECs and U2-OS, and moderate cytotoxicity toward Saos-2. Importantly, the CA4 conjugated copolymers displayed excellent level of antiangiogenic activity as deduced from in vitro endothelial cell tube formation assay using HUVECs. Overall, a novel bone-targeting antiangiogenic polymer-drug conjugate that can be further elaborated to carry additional anticancer drugs is disclosed.

Bisphosphonate-functionalized poly(β-amino ester) network polymers

Three novel bisphosphonate-functionalized secondary diamines are synthesized and incorporated into poly(β-amino ester)s (PBAEs) to investigate the effects of bisphosphonates on biodegradation and toxicity of PBAE polymer networks. These three novel amines, BPA1, BPA2, and BPA3, were prepared from the reactions of 1,4-butanediamine, 1,6-hexanediamine, or 4,9-dioxa-1,12-dodecanediamine with tetraethyl vinylidene bisphosphonate, respectively. The PBAE macromers were obtained from the aza-Michael addition reaction of these amines to 1,6-hexane diol diacrylate (HDDA) and poly(ethylene glycol) diacrylate (PEGDA, M_n  = 575) and photopolymerized to produce biodegradable gels. These gels with different chemistries exhibited similar degradation behavior with mass loss of 53-73% within 24 h, indicating that degradation is mostly governed by the bisphosphonate group. Based on the in vitro cytotoxicity evaluation against NIH 3T3 mouse embryonic fibroblast cells, the degradation products do not exhibit significant toxicity in most cases. It was also shown that PBAE macromers can be used as cross-linkers for the synthesis of 2-hydroxyethyl methacrylate hydrogels, conferring small and customizable degradation rates upon them. The materials reported have potential to be used as nontoxic degradable biomaterials. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 105A: 1412-1421, 2017.

Calcium phosphate nanoparticles functionalized with alendronate-conjugated polyethylene glycol (PEG) for the treatment of bone metastasis

Because of the peculiarity of the bone microstructure, the uptake of chemotherapeutics often happens at non-targeted sites, which induces side effects. In order to solve this problem, we designed a bone-targeting drug delivery system that can release drug exclusively in the nidus of the bone. Alendronate (ALN), which has a high ability to target to hydroxyapatite, was used to fabricate double ALN-conjugated poly (ethylene glycol) 2000 material (ALN-PEG_2k-ALN). The ALN-PEG_2k-ALN was characterized using ¹H NMR and ³¹P NMR and FTIR. ALN-PEG_2k-ALN-modified calcium phosphate nanoparticles (APA-CPNPs) with an ALN targeting moiety and hydrophilic poly (ethylene glycol) arms tiled on the surface was prepared for bone-targeted drug delivery. The distribution of ALN-PEG_2k-ALN was tested by X-ray photoelectron spectroscopy. Isothermal titration calorimetry data indicated that similar to free ALN, both ALN-PEG_2k-ALN and APA-CPNPs can bind to calcium ions. The bone-binding ability of APA-CPNPs was verified via ex vivo imaging of bone fragments. An in vitro release experiment demonstrated that APA-CPNPs can release drug faster in an acid environment than a neutral environment. Cell viability experiments indicated that blank APA-CPNPs possessed excellent biocompatibility with normal cells. Methotrexate (MTX) loaded APA-CPNPs have the same ability to inhibit cancer cells as free drug at high concentrations, while they are slightly weaker at low concentrations. All of these experiments verified the prospective application of APA-CPNPs as a bone-targeting drug delivery system.

Nanoprobes for Multimodal Visualization of Bone Mineral Phase in Magnetic Resonance and Near-Infrared Optical Imaging

Imaging agents with affinity for bone can enable early detection of changes to bone mineral density, which is a hallmark of many bone-associated pathologies such as Paget's disease and osteoporosis. Here, we report the development of a polymer nanoparticle (NP)-based multimodal imaging probe that enables visualization of bone mineral phase in near-infrared (NIR) optical tomography and detection in T₂-weighted magnetic resonance imaging (MRI). Ultrasmall superparamagnetic iron oxide was first encapsulated in NPs derived by blending poly(dl-lactic-co-glycolic acid)-poly(ethylene glycol) (PLGA-PEG) with N-hydroxysuccinimide functionalized-PLGA (NHS-PLGA). Postmodification of NHS surface functionality on the NPs with alendronic acid (Aln), a bone-targeting ligand, yielded stable ultrasmall superparamagnetic iron oxide nanoparticles (USPIONs) containing NPs that exhibit good serum stability and favorable cytocompatibility. These post-Aln-modified NPs exhibit 8- to 10-fold higher affinity for synthetic and biogenic hydroxyapatite in comparison to NPs where Aln was introduced before NP formation and shorten the T₂ relaxation times in both agarose phantoms and fresh spongy bone, thus enabling the interrogation of bone mineral phase in T₂-MRI. Finally, by introducing an NIR-dye-modified PLGA during the NP formation step, NP probes that enable the visualization of bone mineral phase in both NIR optical tomography and MRI have been realized. The system presented herein meets many of the criteria for clinical translation and therefore opens new opportunities for bone imaging and targeted therapeutics.

Bone-targeted delivery of nanodiamond-based drug carriers conjugated with alendronate for potential osteoporosis treatment

This paper describes the design of alendronate-conjugated nanodiamonds (Alen-NDs) and evaluation of their feasibility for bone-targeted delivery. Alen-NDs exhibited a high affinity to hydroxyapatite (HAp, the mineral component of bone) due to the presence of Alen. Unlike NDs (without Alen), Alen-NDs were preferentially taken up by MC3T3-E1 osteoblast-like cells, compared to NIH3T3 and HepG2 cells, suggesting their cellular specificity. In addition, NDs itself increased ALP activity of MC3T3-E1 cells, compared to control group (osteogenic medium) and Alen-NDs exhibited more enhanced ALP activity. In addition, an in vivo study revealed that Alen-NDs effectively accumulated in bone tissues after intravenous tail vein injection. These results confirm the superior properties of Alen-NDs with advantages of high HAp affinity, specific uptake for MC3T3-E1 cells, positive synergistic effect for ALP activity, and in vivo bone targeting ability. The Alen-NDs can potentially be employed for osteoporosis treatment by delivering both NDs and Alen to bone tissue.

Targeted delivery to bone and mineral deposits using bisphosphonate ligands

The high concentration of mineral present in bone and pathological calcifications is unique compared with all other tissues and thus provides opportunity for targeted delivery of pharmaceutical drugs, including radiosensitizers and imaging probes. Targeted delivery enables accumulation of a high local dose of a therapeutic or imaging contrast agent to diseased bone or pathological calcifications. Bisphosphonates (BPs) are the most widely utilized bone-targeting ligand due to exhibiting high binding affinity to hydroxyapatite mineral. BPs can be conjugated to an agent that would otherwise have little or no affinity for the sites of interest. This article summarizes the current state of knowledge and practice for the use of BPs as ligands for targeted delivery to bone and mineral deposits. The clinical history of BPs is briefly summarized to emphasize the success of these molecules as therapeutics for metabolic bone diseases. Mechanisms of binding and the relative binding affinity of various BPs to bone mineral are introduced, including common methods for measuring binding affinity in vitro and in vivo. Current research is highlighted for the use of BP ligands for targeted delivery of BP conjugates in various applications, including (1) therapeutic drug delivery for metabolic bone diseases, bone cancer, other bone diseases, and engineered drug delivery platforms; (2) imaging probes for scintigraphy, fluorescence, positron emission tomography, magnetic resonance imaging, and computed tomography; and (3) radiotherapy. Last, and perhaps most importantly, key structure-function relationships are considered for the design of drugs with BP ligands, including the tether length between the BP and drug, the size of the drug, the number of BP ligands per drug, cleavable tethers between the BP and drug, and conjugation schemes.

Regulation of local bone remodeling mediated by hybrid multilayer coating embedded with hyaluronan-alendronate/BMP-2 nanoparticles on Ti6Al7Nb implants

Hyaluronate-alendronate/BMP-2 nanoparticles were inserted into Gel/Chi multilayers on Ti6Al7Nb for enhancing BMP-2 stability and promoting local osteogenesis under osteoporosis.

Inhibition of fibrous dysplasia via blocking Gsα with suramin sodium loaded with an alendronate-conjugated polymeric drug delivery system

A bone-targeted polymeric drug delivery system was constructed to inhibit fibrous dysplasia efficiently via blocking Gsα with suramin sodium.

Targeting tumor metastases: Drug delivery mechanisms and technologies

Primary sites of tumor are the focal triggers of cancers, yet it is the subsequent metastasis events that cause the majority of the morbidity and mortality. Metastatic tumor cells exhibit a phenotype that differs from that of the parent cells, as they represent a resistant, invasive subpopulation of the original tumor, may have acquired additional genetic or epigenetic alterations under exposure to prior chemotherapeutic or radiotherapeutic treatments, and reside in a microenvironment differing from that of its origin. This combination of resistant phenotype and distal location make tracking and treating metastases particularly challenging. In this review, we highlight some of the unique biological traits of metastasis, which in turn, inspire emerging strategies for targeted imaging of metastasized tumors and metastasis-directed delivery of therapeutics.

Biodistribution of Fracture-Targeted GSK3β Inhibitor-Loaded Micelles for Improved Fracture Healing

Bone fractures constitute a major cause of morbidity and mortality especially in the elderly. Complications associated with osteoporosis drugs and the age of the patient slow bone turnover and render such fractures difficult to heal. Increasing the speed of fracture repair by administration of a fracture-targeted bone anabolic agent could find considerable application. Aspartic acid oligopeptides are negatively charged molecules at physiological pH that adsorb to hydroxyapatite, the mineral portion of bone. This general adsorption is the strongest where bone turnover is highest or where hydroxyapatite is freshly exposed. Importantly, both of these conditions are prominent at fracture sites. GSK3β inhibitors are potent anabolic agents that can promote tissue repair when concentrated in a damaged tissue. Unfortunately, they can also cause significant toxicity when administered systemically and are furthermore difficult to deliver due to their strong hydrophobicity. In this paper, we solve both problems by conjugating the hydrophobic GSK3β inhibitor to a hydrophilic aspartic acid octapeptide using a hydrolyzable bond, thereby generating a bone fracture-targeted water-soluble form of the drug. The resulting amphiphile is shown to assemble into micelles, extending its circulation time while maintaining its fracture-targeting abilities. For measurement of pharmacokinetics, an 125I was introduced at the location of the bromine in the GSK3β inhibitor to minimize any structural differences. Biodistribution studies demonstrate a greater than 4-fold increase in fracture accumulation over healthy bone.

Well-designed bone-seeking radiolabeled compounds for diagnosis and therapy of bone metastases

Bone-seeking radiopharmaceuticals are frequently used as diagnostic agents in nuclear medicine, because they can detect bone disorders before anatomical changes occur. Furthermore, their effectiveness in the palliation of metastatic bone cancer pain has been demonstrated in the clinical setting. With the aim of developing superior bone-seeking radiopharmaceuticals, many compounds have been designed, prepared, and evaluated. Here, several well-designed bone-seeking compounds used for diagnostic and therapeutic use, having the concept of radiometal complexes conjugated to carrier molecules to bone, are reviewed.

A delivery system specifically approaching bone resorption surfaces to facilitate therapeutic modulation of microRNAs in osteoclasts

Dysregulated microRNAs in osteoclasts could cause many skeletal diseases. The therapeutic manipulation of these pathogenic microRNAs necessitates novel, efficient delivery systems to facilitate microRNAs modulators targeting osteoclasts with minimal off-target effects. Bone resorption surfaces characterized by highly crystallized hydroxyapatite are dominantly occupied by osteoclasts. Considering that the eight repeating sequences of aspartate (D-Asp8) could preferably bind to highly crystallized hydroxyapatite, we developed a targeting system by conjugating D-Asp8 peptide with liposome for delivering microRNA modulators specifically to bone resorption surfaces and subsequently encapsulated antagomir-148a (a microRNA modulator suppressing the osteoclastogenic miR-148a), i.e. (D-Asp8)-liposome-antagomir-148a. Our results demonstrated that D-Asp8 could facilitate the enrichment of antagomir-148a and the subsequent down-regulation of miR-148a in osteoclasts in vivo, resulting in reduced bone resorption and attenuated deterioration of trabecular architecture in osteoporotic mice. Mechanistically, the osteoclast-targeted delivery depended on the interaction between bone resorption surfaces and D-Asp8. No detectable liver and kidney toxicity was found in mice after single/multiple dose(s) treatment of (D-Asp8)-liposome-antagomir-148a. These results indicated that (D-Asp8)-liposome as a promising osteoclast-targeting delivery system could facilitate clinical translation of microRNA modulators in treating those osteoclast-dysfunction-induced skeletal diseases.

Bisphosphonate–polyaspartamide conjugates as bone targeted drug delivery systems

Poly-hydroxy-aspartamide was used as a backbone to synthesize bisphosphonate derivatives thus achieving macromolecular carriers to be potentially used as targeting agents for bone drug delivery.

Bone-targeted acid-sensitive doxorubicin conjugate micelles as potential osteosarcoma therapeutics

Osteosarcoma is a malignancy of the bone that primarily affects adolescents. Current treatments retain mortality rates, which are higher than average cancer mortality rates for the adolescent age group. We designed a micellar delivery system with the aim to increase drug accumulation in the tumor and potentially reduce side effects associated with chemotherapy. The design features are the use of the hydrophilic D-aspartic acid octapeptide as both the effective targeting agent as well as the hydrophilic micelle corona. Micelle stabilization was accomplished by binding of model drug (doxorubicin) via an acid-sensitive hydrazone bond and incorporating one to four 11-aminoundecanoic acid (AUA) moieties to manipulate the hydrophobic/hydrophilic ratio. Four micelle-forming unimers have been synthesized and their self-assembly into micelles was evaluated. Size of the micelles could be modified by changing the architecture of the unimers from linear to branched. The stability of the micelles increased with increasing content of AUA moieties. Adsorption of all micelles to hydroxyapatite occurred rapidly. Doxorubicin release occurred at pH 5.5, whereas no release was detected at pH 7.4. Cytotoxicity toward human osteosarcoma Saos-2 cells correlated with drug release data.

Engineering of cell membranes with a bisphosphonate-containing polymer using ATRP synthesis for bone targeting

The field of polymer-based membrane engineering has expanded since we first demonstrated the reaction of N-hydroxysuccinimide ester-terminated polymers with cells and tissues almost two decades ago. One remaining obstacle, especially for conjugation of polymers to cells, has been that exquisite control over polymer structure and functionality has not been used to influence the behavior of cells. Herein, we describe a multifunctional atom transfer radical polymerization initiator and its use to synthesize water-soluble polymers that are modified with bisphosphonate side chains and then covalently bound to the surface of live cells. The polymers contained between 1.7 and 3.1 bisphosphonates per chain and were shown to bind to hydroxyapatite crystals with kinetics similar to free bisphosphonate binding. We engineered the membranes of both HL-60 cells and mesenchymal stem cells in order to impart polymer-guided bone adhesion properties on the cells. Covalent coupling of the polymer to the non-adherent HL-60 cell line or mesenchymal stem cells was non-toxic by proliferation assays and enhanced the binding of these cells to bone.